Why did Blanche get fired from her teaching job?

Blanche was fired because she was involved in an inappropriate relationship with one of her students.

She's not going back to teach school! In fact I am willing to bet you that she never had no idea of returning to Laurel! She didn't resign temporarily from the high school because of her nerves! No, siree, Bob! She didn't. They locked her out of that high school before the spring term ended-and I hate to tell you the reason that step was taken! A seventeen-year-old boy--she'd gotten mixed up with!